This invention relates to the production of low-expansion, thin-walled cordierite ceramic honeycomb structures from magnesia, alumina, and silicate source materials. More particularly, the invention relates to an improved extrusion method for forming high quality, thin-walled honeycomb structures free of circumferential skin cracks.
The use of honeycomb extrusion dies to form cellular ceramic honeycombs of cordierite composition and very low thermal expansion from plasticized mixtures of ceramic batch materials is described in U.S. Pat. Nos. 3,790,654 and 3,885,977. Such honeycombs remain in widespread commercial use as catalyst supports for emissions control applications such as automotive exhaust treatment systems.
The large body of prior art pertaining to cordierite honeycomb extrusion includes honeycomb extrusion dies of many different constructions and modes of operation, all being generally designed to form cordierite honeycombs having a webbed cellular or channeled core structure surrounded in most cases by a smooth integral outer skin layer. Typically, the honeycomb extrusion dies employed are multi-component assemblies including at least a web-forming die body combined with a skin-forming mask or mask assembly attached to the outlet side of the die body. The die body incorporates inlet channels or feedholes leading to an intersecting array of discharge slots on the discharge face of the die through which the batch material is extruded as crisscrossing webs forming the central channeled core structure of the honeycomb. The mask is generally a ring-like circumferential structure in the form of a collar defining the periphery of the honeycomb and against which the outer skin layer of the honeycomb is formed.
Many of the known prior art die constructions are designed specifically to overcome the interrelated problems of poor skin adherence to the webbed honeycomb core and distortion of the peripheral webs of the core as the skin is joined therewith during the extrusion process. Skin adherence requires the development or maintenance of at least some convergent bonding pressure between the extruding skin and the webs forming the peripheral cell walls, but too much pressure results in a distortion of the peripheral webs and cells of the honeycomb.
U.S. Pat. No. 4,349,329 discloses an extrusion die particularly designed to minimize peripheral cell distortion. In that design, batch material for the skin is collected in a pooling zone beneath the skin-forming mask and is then extruded through a gap between the die body and mask to join with an extruding webbed honeycomb core portion issuing from the die body. The core portion features thickened peripheral webs forming channel walls that resist distortion as the skin joins the core portion during extrusion. U.S. Pat. No. 5,219,509 describes another die design wherein a separate stream of batch material for the skin flowing inwardly from a peripheral collection zone beneath the mask is redirected by the mask and die body onto a flow path nearly parallel with but converging slightly toward the honeycomb extrusion axis of the die. This design also minimizes the distortion of web portions of the peripheral cells, in this case by limiting the skin pressure applied to the cells.
The art is also aware of various means for controlling the thickness of the extruded skin. U.S. Pat. Nos. 4,668,176 and 4,710,123, for example, describe die designs wherein skin thickness can be controlled by controlling the width of the gap formed between the die body and mask. Also shown are means for adjusting the supply of batch material to the skin-forming section of the die to maintain the material flow needed to provide the selected skin thickness.
A common feature of the extrusion process as practiced in the prior art is that of extruding the plasticized batch material at batch pressures and viscosities where the speed of skin extrusion will substantially match the speed of honeycomb core extrusion. Published Japanese patent application JP 61-005915, for example, discloses die modifications expressly designed to properly match the core extrusion rate with the skin extrusion rate. Matching is viewed as important because excessive shear between the webbed core and skin sections of the extruding honeycomb at the site of the core-skin interface has been thought to contribute to reduced skin adherence, increased peripheral web distortion, or both.
Tightening emissions control regulations, particularly for automobiles, require honeycombs of substantially decreased wall thickness and increased channel density for improved catalytic efficiency. For example, the demand for ultra-thin-wall honeycombs, defined for present purposes as honeycombs having channel wall or web thicknesses of 0.004 inches (0.10 mm) or less, is increasing substantially. At the same time, honeycombs incorporating from 600-1200 channels/square inch (about 90-190/cm2) are entering advanced development or commercial use.
Although current and emerging extrusion die designs can be adapted to the extrusion of ultra-thinwall honeycombs with no gross forming defects, new problems unique to these fine-structured products still must be solved. One such problem relates to skin defects referred to as xe2x80x9choop cracksxe2x80x9d, which are cracks transverse to the channel axis of the honeycombs that appear to develop within the skin layers of the ultra-thinwall honeycombs only during firing. These cracks, which can occur at multiple points along the length of a cracked honeycomb and extend over significant portions of the honeycomb circumference, render the products unacceptable for sale. Extensive experiments designed by us to identify the origin of this cracking behavior established that the cracks developed during the cooling phase of the firing cycle.
Conventional extrusion methods and apparatus that can produce defect-free fired honeycombs at conventional skin and web thicknesses tend to produce hoop-cracked fired ware with certain batch mixtures at ultra-thinwall web thicknesses. These cracks develop even though no incipient or partially developed cracking defects have been detected in the wet or dried green extruded honeycombs from which the fired honeycombs are made.
Extensive experiments designed by us to identify the origin of this cracking behavior have established that the circumferential hoop cracks in the skins of ultra-thinwall honeycombs develop during the cooling phase of the firing cycle. Based on this finding and further analysis, this crack development has been correlated with a mis-alignment of high-aspect-ratio batch particles that develops in the extruding skin layer during the extrusion process.
The present invention achieves the elimination of this hoop cracking by improving the alignment of batch particles in the skin so that the degree of alignment achieved is similar to the degree of alignment observed in the webbed core portions of the honeycombs. This result is obtained by changing the way in which the skin layers of the ultra-thinwall honeycombs are extruded.
In a key aspect, therefore, the invention resides in an improved extrusion method through which a crack-free honeycomb body having a central webbed core structure and a peripheral outer layer or skin may be formed. The generalized method involves the known fundamental steps of extruding a plasticized powder batch material through a honeycomb extrusion die assembly. The die assembly comprises a die body and a skin-forming mask, and the basic steps of the method are analogous to those employed for the production of conventional honeycombs.
The core or central webbed structure of the honeycomb is conventionally formed by batch material extruded through a criss-crossing array of discharge slots on the outlet face of the die body, these slots being supplied with batch material via feedholes through the body connecting the slots with a pressurized supply of batch material in a batch reservoir behind the inlet face of the die. The peripheral skin layer of the honeycomb is formed by batch material extruded through a skin-forming gap provided between the mask of the die and the outlet face thereof.
To achieve the alignment of high aspect ratio particles in the skin that is required to provide crack-free ultra-thinwall products in accordance with the invention, the relative speeds at which the webbed core and peripheral skin are extruded through the skin-forming gap in the die should be controlled such that the extrusion speed of the skin (VS) through the skin-forming gap is less than the extrusion speed of the webbed core (VW) exiting the die discharge slots. The difference in extrusion speeds is closely controlled. We have determined that good particle alignment can be assured if the extrusion speed of the skin through the gap is in the range of 50-95% of the speed of extrusion of the core portion of the honeycomb. This is in contrast to the matching of core and skin extrusion speeds practiced in the prior art.
Maintaining an extrusion speed differential with a higher core than skin extrusion rate causes the extruding skin layer to be drawn out, over the interval between the point of its formation at the skin-forming gap of the extrusion die and the point at which it reaches speed parity with the webbing of the honeycomb core. At the latter point the skin reaches an equilibrium thickness; therefore an equivalent indicator of good particle alignment in the skin is the relationship between the width of the skin-forming gap defining the initial skin layer thickness and the ultimate thickness of the skin on the fully extruded honeycomb. The formation of a peripheral skin layer on the extruded honeycomb having a thickness in the range of 50-95% of the width of the skin-forming gap will insure that good alignment of high-aspect-ratio batch particles in the skin will be achieved.
Any of a variety of extrusion control strategies may be used to provide useful differentials in core and skin extrusion rates, or effective skin elongation and thinning, during the extrusion process. A common characteristic of these strategies is the avoidance, in ultra-thinwall honeycomb manufacture, of any extrusion condition that results in any thickening or compaction of the skin-forming layer after it is discharged from skin-forming gap and during its bonding with the webbed core. We have found that skin extrusion at rates equal to or higher than the corresponding core extrusion rates appears to compact the skin layer and thereby substantially increase the likelihood of encountering hoop cracking in the finished product.
Without intending to be bound by theory, it is presently thought that the particle realignments that can occur at matching or excessive skin extrusion rates substantially increase thermal expansion differentials between the core and skin sections of fired ultra-thinwall honeycombs. These thermal expansion differentials could cause hoop cracking in the higher expansion skins as the honeycombs are cooled to ambient temperatures after firings.
The product of the process as above described is a thin-walled honeycomb product that is free of circumferential skin cracks as made, and more likely to resist thermal damage in use than conventionally extruded honeycombs of similar geometry. These advantages are derived from the fact that the degree of cordierite crystal alignment in the honeycomb skins is substantially the same as the degree of cordierite crystal alignment in the honeycomb webs. In another aspect, then, the invention includes an extruded cordierite honeycomb body of thin-walled geometry with improved thermal durability as well as improved skin integrity.
In geometry, the cordierite honeycombs of the invention include a webbed core section comprising webs having a web thickness not exceeding about 0.004 inches, and a crack-free extruded skin layer disposed on the webbed core section. The extruded skin layer will have a thickness of at least twice the thickness of the webs forming the honeycomb core. More typically, the skin thickness will be 3-10 times the web thickness.
Characteristic of these honeycombs, the extent of cordierite crystal alignment obtained in the honeycomb skins is close to or substantially matches the degree of cordierite crystal alignment developed in the webs of honeycomb core. The degree of cordierite crystal alignment in each of the skin and web sections of the honeycombs may be determined in the known manner from x-ray diffraction analyses of the two sections. Thus, for these honeycombs, the x-ray diffraction I-ratios (IR), determined in the known manner for each of the skin and web sections from the formula:       I    R    =            I              (        110        )                            I                  (          110          )                    +              I                  (          002          )                    
will be similar or substantially the same in value.